What’s been behind the strange drop in American body temperatures over the last 200 years?

The human body is often said to rest at a healthy internal temperature of 37 degrees Celsius or 98.6 degrees Fahrenheit.

This average was established two centuries ago in France, and yet it seems that our ‘normal’ physiology has changed a little in the meantime.

At the beginning of last year, researchers in the United States combed civil war veteran records and national health surveys and found that temperatures among men born at the beginning of this century were 0.59 degrees Celsius cooler than men born about two hundred years earlier.

Women, on the other hand, had seen a drop of 0.32 degrees Celsius since the 1890s.

At the time, the authors suggested that it might have something to do with inflammation due to disease, which is closely linked to body temperature. With the advent of modern medicine, we have seen a decline in chronic infections, and perhaps, the authors suggested, this has, so to speak, cooled us.

Later in 2020, another group of researchers found an eerily similar reduction in body temperature among a relatively remote indigenous tribe in Bolivia, where infections have remained widespread and medical treatment minimal, despite some modern changes.

The reasons for the recent drop in body temperature clearly had to go beyond improved hygiene, cleaner water or improved medical care, and some researchers at Harvard are now investigating another explanation: a drop in physical activity.

When a person exercises regularly, it often coincides with an increase in their metabolism. This in turn can raise their body’s resting temperature for hours or even up to a day, meaning that declining body temperature measurements may indicate declining physical activity.

Unfortunately, the methods we have for measuring physical activity today were not about 200 years ago, so we can not really compare how we move now with how we moved then.

What could be possible, however, is to use historical body temperature data as a “thermometer” to measure physical activity before we started keeping track of these things.

If we can model the relationship between physical activity, metabolism and body temperature, we could theoretically work backwards.

The idea started as a “back-of-the-envelope” calculation among Harvard researchers, and although their “first pass estimate” is a good start, it is still based on a lot of assumptions. That said, it’s an intriguing hypothesis.

The model that the researchers ultimately created found that each 1 ° C increase in historical body temperature is associated with a change of about 10 percent in resting metabolism.

Considering how much men’s body temperatures appear to have dropped since the 1820s, their metabolism must therefore have dropped by 6 percent over the same period.

That equates to about half an hour of physical activity a day, according to the authors’ calculations. More precisely, a 27-minute brisk walk or slow run for a 75-pound (165-pound) man.

“This is a first-time estimate for taking physiological data and trying to quantify declines in activity,” explains skeletal biologist Andrew Yegian of Harvard University.

“The next step would be to try to use this as a tool for other populations.”

Because these initial estimates use body temperature as a proxy for metabolic activity and then metabolic activity as a proxy for physical activity, it is highly unlikely that these results are not really representative of reality.

The rate at which a population converts calories can be determined for more than just physical activity, though it is undoubtedly true that the average American today trains less than they did 50 years ago, thanks to cars, television and the beginning of desk work.

It’s just less clear what it does to our metabolism and the temperature of our bodies. And it may not be the same for men and women.

“Fat also acts as an insulator, affecting heat dissipation from the body, while increasing the cost of PA, and our estimation methods did not correct for changes in fat mass over time,” the authors write.

A reduced need to thermoregulate in modern environments can also affect our metabolism, as well as improved health and nutrition.

The authors admit that their calculations need further refinement, but they hope that their approximation will serve “as an anchor to understand how the decline in physical activity affected health and morbidity in the industrial era.”

The study was published in Current biology.


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